One of the major problems of cancer chemotherapy is the existence of drug resistance in tumors resulting in reduced responsiveness to chemotherapy. Some human cancers, e.g. kidney and colon carcinoma, are drug resistant before treatment begins, while in others drug resistance develops over successive rounds of chemotherapy. One type of drug resistance, called multidrug resistance, is characterized by cross resistance to functionally and structurally unrelated drugs. Typical drugs that are affected by the multidrug resistance are doxorubicin, vincristine, vinblastine, colchicine and actinomycin D, and others. At least some multidrug resistance is a complex phenotype which has been linked to a high expression of a cell membrane drug efflux transporter called Mdr1 protein, also known as P-glycoprotein. This membrane "pump" has broad specificity and acts to remove from the cell a wide variety of chemically unrelated toxins. (See Endicott, J. A., et al. "The Biochemistry of P-Glycoprotein-Mediated Multidrug Resistance", Ann. Rev. Biochem. Vol. 58, pgs. 127-71, 1989.)
Recently, a similar mechanism of a broad spectrum drug resistance has been reported for certain microorganisms. These results indicate the existence of bacterial efflux systems of extremely broad substrate specificity that is similar to the multidrug resistance pump of mammalian cells. (See Nikaido, N., "Prevention of Drug Access to Bacterial Targets: Permeability Barriers and Active Efflux", Science, Vol. 264, pgs. 382-388, 1994; and Gottesman, M. M., et al. Annual Rev. Biochem., Vol 62, 385 1993)
Substances which reverse multidrug resistance are known as resistance modification agents (RMAs), and are of importance in potentiating the cytotoxicity of chemotherapeutic agents to which a human cancer has become resistant. Although many agents have been identified as RMAs in vitro, a large proportion have little or no therapeutic potential because of high toxicity in vivo at the doses required to reverse multidrug resistance. For example, metabolic poisons, such as azide, reverse multidrug resistance in vitro but have no usefulness in vivo. Most other highly effective RMAs, such as PSC833, appear to work as competitive antagonists of a drug binding site on the Mdr1 protein. Many of these agents also have toxicity which limits their usefulness in vivo. Consequently, there is a need to develop alternate pharmacological strategies for reversing multidrug resistance to provide RMAs with improved activity and lower overall toxicity.
Decreased intracellular drug accumulation through overexpression of the drug efflux Mdr1 protein is important to, but apparently not the only factor, in the multidrug resistance phenotype. Altered intracellular drug distribution and binding, among other possibilities, also seem to play a role. For example, the mechanism of reversing doxorubicin resistance using verapamil appears to be more related to altered intracellular distribution of doxorubicin than increased accumulation in the cell, as detailed in Schuurhmuis, G. J., et al., "Quantitative determination of factors contributing to doxorubicin resistance in multidrug resistant cells," J. Natl. Cancer Inst., 81:1887-1892, 1989. In that report, it is shown that doxorubicin is concentrated almost exclusively in the nucleus in drug sensitive cells, and mainly in the cytoplasm in drug resistant cells. With the addition of verapamil, doxorubicin is localized mainly in the nucleus in drug resistant cells. Thus, high affinity binding of drugs to Mdr1 does not appear to be sufficient for optimal efflux, suggesting the existence of additional, rate limiting steps which may be susceptible to pharmacological intervention.
Certain non-ionic amphipathic surfactants, such as Tween 80 and CREMOPHOR.RTM. EL, have evidenced RMA activity. (See Riehm H., et al. "Potentiation of drug effect by Tween 80 in Chinese hamster cells resistant to actinomycin D and Danomycin" Cancer Res. Vol. 32, pgs. 1195-1200, 1972 and Woodcock, D. B., et al., "Reversal of the multidrug resistance phenotype with CREMOPHOR.RTM. EL, a common vehicle for water-insoluble vitamins and drugs" Cancer Res. Vol. 50, pgs. 4199-4203, 1990.) However, Tween 80 potentiates drug toxicity in both parental and multidrug resistant cells, calling into question the specificity of the Tween 80 effect on multidrug resistance. An effect on drug efflux has not been demonstrated. CREMOPHOR.RTM. EL is a complicated mixture of polyoxyethylated esters of triglycerides of mainly ricinoleic acid (castor oil), the composition and RMA active component of which have not been identified. Use of CREMOPHOR.RTM. EL in vivo is complicated by adverse histamine release in some patients. There have also been reports of neurotoxicity associated with the administration of CREMOPHOR.RTM. EL.
Cancer chemotherapy with cytotoxic agents can be successful only if the tumor cells are more sensitive than normal cells whose destruction is incompatible with survival of the host. Success, defined either as cure or clinically significant remission, is not readily explained by the still popular idea that tumor cells are more susceptible to cytotoxic agents because they are dividing more rapidly than vital normal cells, e.g. hematopoietic precursor cells. That rapid proliferation does not wholly account for the selective drug sensitivity of tumors is demonstrated by the common observations that some drug-sensitive cancers are not rapidly dividing, and that many rapidly proliferating tumors exhibit resistance. To say that the mechanisms accounting for the success or failure of chemotherapy for most human tumors is incompletely understood today is undoubtedly an understatement.
However, recent evidence suggests that the selectivity of chemotherapy for the relatively few tumors ever cured by drugs depends, to a large extent, upon their easy susceptibility to undergo apoptosis, i.e. to kill themselves. Many cytotoxic drugs that kill cells by crippling cellular metabolism at high concentration can trigger apoptosis in susceptible cells at much lower concentration. This appears to account for the unusual chemosensitivity of many lymphoid tumors, since many normal lymphocytes are "primed" to undergo self destruction as an essential part of the mechanism for generating and controlling diversity of the immune response. Increased susceptibility to apoptosis may also be acquired by tumor cells as a byproduct of the genetic changes responsible for malignant transformation. For example, tumor cells with constitutive c-myc expression may undergo apoptosis in response to DNA damage by anticancer agents, whereas normal cells are able to pause at checkpoints in the cell cycle to repair the damage, or may not be cycling at all, rendering them highly resistant to apoptosis in this setting.
Although some tumors are composed of cells which are highly sensitive to apoptotic stimuli, and others have a genetic changes which, in isolation, predispose to apoptosis, most tumors tend to acquire other genetic lesions which abrogate this increased sensitivity. Either at presentation or after therapeutic attempts, the tumor cells actually become less sensitive to apoptosis than vital normal dividing cells. Such tumors are generally not curable by available chemotherapeutic approaches. Decreased apoptotic response has been associated with increased malignant potential and has been shown to confer pleiotropic drug resistance by oncogene transfer techniques. Therefore, although previously recognized mechanisms of drug resistance, such as decreased drug uptake, altered intracellular drug localization, accelerated detoxification and alteration of drug target continue to be regarded as important factors, pleiotropic resistance due to defective apoptotic response has recently emerged as a distinct and significant category of drug resistance in cancer. Resistance due to failure to trigger apoptosis is sometimes called "downstream" drug resistance to distinguish it from the "classical" mechanisms mentioned above.
Thus, what is needed is a clearly identified class of compositions that reverse multidrug resistance in vivo. The composition should have a low occurrence of adverse side-effects. In addition, what is further needed is a composition and method for stimulating apoptosis in cancer cells.